LSSD circuits are attractive for use as the size and complexity of integrated circuit chips increase. The increase in density of the VLSI circuits has led to improved methods of testing and circuits which are designed and configured to enhance their testability. Although LSSD circuits have many attractive features relating to testability, their response to input signals is limited. In LSSD, the level of an input signal, whether high or low, dictates the state that a receiving latch will take when it is clocked. The transition of the signal or edge is not important in an LSSD design, as all signals are stable to the system's synchronizing clock. With an asynchronous signal which can change at any time within a clock cycle, the data setup and hold time specifications can be violated, thus causing oscillations or metastability to occur in the latch. Metastability is the condition where the latch is changing its state when the clock is turned off, but is neither a one or a zero. This problem is not unique to LSSD designs as other clocked logic designs have their own data setup and hold time specifications.
The Micro Channel (TM) is a complex asynchronous bus currently used in PS/2 (TM) and other low end processor systems. The Micro Channel contains a 32-bit address bus, 32-bit data bus, an arbitration bus, and a variety of control signals. These control signals are in part to validate both the address and/or data busses. In an edge-triggered TTL MSI environment, these control signals can be useful in reliably latching the busses. However, in chip designs where LSSD rules and guidelines must be followed, using these control signals in an asynchronous fashion can make for difficult clocking and timing situations. These time situations can become even more complex when extensive use of asynchronous clocking and state machining is utilized. Thus, interfacing a chip design which conforms to LSSD to asynchronous signal requires that those asynchronous signals being received be synchronized to the receiving system clock before they are acted upon.